Many new and potentially useful technologies are being developed which may form the basis of future medical cures and therapies. Examples of such technologies include, gene replacement, antisense gene therapy, triplex gene therapy and ribozyme-based therapy. However, to be successful, these technologies require effective means for the delivery of the therapeutic agent across cellular, nuclear and microorganismal membranes.
The recent advent of technology, and advances in our ability to understand the structure and function of many genes makes it possible to selectively turn off or modify the activity of a given gene. Alteration of gene activity can be accomplished many ways. For example, oligonucleotides that are complementary to certain gene messages or viral sequences, known as “antisense” compounds, have been shown to have an inhibitory effect against viruses. By creating an antisense compound that hybridizes with the targeted RNA message of cells or viruses the translation of the message into protein can be interrupted or prevented. In this fashion gene activity can be modulated.
The ability to deactivate specific genes provides great therapeutic benefits. For example, it is theoretically possible to fight viral diseases with antisense RNA and DNA molecules that seek out and destroy viral gene products. In tissue culture, antisense oligonucleotides have inhibited infections by herpes-viruses, influenza viruses and the human immunodeficiency virus that causes AIDS. It may also be possible to target antisense oligonucleotides against mutated oncogenes. Antisense technology also holds the potential for regulating growth and development. However, in order for the gene therapy to work, antisense therapeutic compounds must be delivered across cellular plasma membranes to the cytosol.
Gene activity is also modified using sense DNA in a technique known as gene therapy. Defective genes are replaced or supplemented by the administration of “good” or normal genes that are not subject to the defect. The administered normal genes which insert into a chromosome, or may be present in extracellular DNA, produce normal RNA, which in turn leads to normal gene product. In this fashion gene defects and deficiencies in the production of gene product may be corrected. Still further gene therapy has the potential to augment the normal genetic complement of a cell. For example, it has been proposed that one way to combat HIV is to introduce into an infected person's T cells a gene that makes the cells resistant to HIV infection. This form of gene therapy is sometimes called “intracellular immunization.” Genetic material such as polynucleotides may be administered to a mammal to elicit an immune response against the gene product of the administered nucleic acid sequence. Such gene vaccines elicit an immune response in the following manner. First, the nucleic acid sequence is administered to a human or animal. Next, the administered sequence is expressed to form gene product within the human or animal. The gene product inside the human or animal is recognized as foreign material and the immune system of the human or animal mounts an immunological response against the gene product. However, this approach currently is not feasible due to a lack of effective gene delivery systems that facilitate the delivery of genetic material across both cellular and nuclear membranes.
Finally, gene therapy may be used as a method of delivering drugs in vivo. For example, if genes that code for therapeutic compounds can be delivered to endothelial cells, the gene products would have facilitated access to the blood stream. Currently, genes are delivered to cells ex vivo and then reintroduced to the animal.
Retroviral vectors can be used to deliver genes ex vivo to isolated cells, which are then infused back into the patient. However, retroviral vectors have some drawbacks, such as being able to deliver genes only to dividing cells, random integration of the gene to be delivered, potentially causing unwanted genetic alterations, and possibly reverting back to an infectious wild-type retroviral form. Another drawback of antisense gene therapy is that it is effective at the messenger RNA level, which means that antisense oligonucleotides must be introduced in a quantity to interact with all or a substantial number of the mRNA in the cytosol, and that such treatment is only effective during active synthesis of mRNA. Further, the oligonucleotides must be maintained at this high quantity level throughout mRNA synthesis to be effective over time.
Newly developed “triplex DNA” technology represents an improvement in gene regulation. Triplex DNA technology utilizes oligonucleotides and compounds that specifically bind to particular regions of duplex DNA, thereby inactivating the targeted gene. An advantage of triplex DNA technology is that only a single copy of the oligonucleotide or compound is required to alter gene expression because the binding is at the DNA level, not the mRNA level. A drawback of triplex DNA technology, however, is that the oligonucleotide or compound must pass through not only the cellular membrane, but also the microbial membrane in the case of treating microbial infections, or the nuclear membrane in the case of altering eukaryotic gene function or expression of foreign DNA integrated into chromosomal DNA.
Another emerging technology relates to the therapeutic use of ribozymes for the treatment of genetic disorders. Ribozymes are catalytic RNA molecules that consist of a hybridizing region and an enzymatic region. Ribozymes may in the future be engineered so as to specifically bind to a targeted region of nucleic acid sequence and cut or otherwise enzymatically modify the sequence so as to alter its expression or translation into gene product.
There is a great need, therefore, for improved delivery systems for genetic material such as genes, polynucleotides, and antisense oligonucleotides that can be used in gene therapy. More specifically, there is a need for non-toxic compositions having surfactant properties that can facilitate the transport of genetic compounds and other drugs and therapeutic compounds across cellular membranes.
There is a particularly urgent need for an effective treatment for Acquired Immune Deficiency Syndrome, or AIDS, a disease thought to be caused by a human retrovirus, the Human T Lymphotropic Virus III (HTLV-III) which is also called human immunodeficiency virus or HIV. Like other retroviruses, HIV has ribonucleic acid, or RNA, as its genetic material. When the virus enters the host cell, a viral enzyme called reverse transcriptase exploits the viral RNA as a template to assemble a corresponding molecule of DNA. The DNA travels through the cell nucleus and inserts itself among the host chromosomes, where it provides the basis for viral replication.
In the case of HIV, the host cell is often a T4 lymphocyte, a white blood cell that has a central and regulatory role in the immune system. Once it is inside a T4 cell, the virus may remain latent until the lymphocyte is immunologically stimulated by a secondary infection. Then the virus reproducing itself rapidly killing or rendering ineffective the host cell. The resulting depletion of the T4 cells, and loss of activity leaves the patient vulnerable to “opportunistic” infections by an agent that would not normally harm a healthy person. The virus damages the host by many other mechanisms as well.
Many therapies against AIDS infection that are currently being investigated. Several of these therapies under investigation are based on interrupting the reverse transcriptase as it assembles the viral DNA destined to become the virus. The drugs used for this purpose are chemical analogs of the nucleic acids that form the subunits of DNA. When the analog is supplied to an infected cell, reverse transcriptase will incorporate it into a growing DNA chain. Because the analog lacks the correct attachment point for the next subunit, however, the chain is terminated. The truncated DNA cannot integrate itself into the host chromosomes or provide the basis for viral replication, and so the spread of the infection is halted. One of the compounds that is thought to act by mimicking a nucleotide is azidothymidine, or AZT. However, AZT is known to have serious side effects and its efficacy in mitigating the AIDS disease has been questioned. The efficacy of AZT and other antiviral and antimicrobial drugs could be increased if improved means and methods for delivering therapeutic agents to the site of infection were available.